Conjugation of agents having a known or potential therapeutic benefit to water soluble, non-immunogenic polymers can impart to the agent desirable characteristics including, among others, improved solubility, greater stability, and reduced immunogenicity. It has become increasingly important in the pharmaceutical industry to develop conjugates having these characteristics so as to increase the number of options for therapeutic benefit.
An example of a polymer that can be used to develop conjugates with therapeutic agents is poly(ethylene glycol) (“PEG”). Therapeutic agents conjugated to PEG are sometimes said to be “PEGylated.” Several PEGylated therapeutics have been developed that exhibit enhanced water solubility, longer circulation lifetimes, and lower immunogenicity as compared to the unconjugated therapeutic agent. Because of the rapid motion and heavy hydration of the polymer, PEGs usually are of much higher apparent molecular weight than the therapeutics to which they are attached. Thus, they tend to mask the therapeutic agent from the immune system and to preclude excretion through kidneys.
The term PEG is commonly used to describe any of a series of polymers having the general formula HO—(CH2CH2—O)n—H, where “n” represents the number of ethylene oxide monomers in the polymer. However, the parent polymer is generally unsuitable for attachment to a therapeutic agent. Hydroxyl groups are relatively unreactive toward groups commonly present on therapeutic agents and thus PEG normally has to be “activated” by converting at least one end hydroxyl group into a more reactive form. It is also usually important to activate the PEG polymer with a terminal group that is selective in its reactions. For example, several PEG derivatives have been developed that are more likely to react with amine groups. Others have been developed that preferentially react with thiol groups.
Successful PEG derivatives may have to meet a number of requirements, depending on the specific application. For conjugation to proteins, the PEG derivative should usually have a desirable and suitably selective reactivity at physiologic conditions of temperature, pressure, and pH to preserve the activity of the unconjugated protein. In some circumstances, it is desirable to cleave the PEG polymer from the therapeutic agent at some point after the agent is delivered in vivo.
Some PEG derivatives have been used in combination with other polymers to prepare insoluble gels in which drugs can be entrapped or chemically bound. For example, Sawhney et al., Macromolecules, 26:581 (1993) describes the preparation of block copolymers of PEG with polyglycolide or polylactide blocks at both ends of the PEG chain. The copolymers are then activated by terminal substitution with acrylate groups, as shown below.CH2═CH—CO—(O—CH2—CO)n—O-PEG-O—(CO—CH2—O)n—CO—CH═CH2 
In the above formula, the glycolide blocks are the —O—CH2—CO— units. The addition of a methyl group to the methylene gives a lactide block; n can be multiples of 2. Vinyl polymerization of the acrylate groups produces an insoluble, crosslinked gel with a polyethylene backbone.
The polylactide or polyglycolide segments of the polymer backbone shown above, which are ester groups, are susceptible to slow hydrolytic breakdown, with the result that the crosslinked gel undergoes slow degradation and dissolution. The hydrogel degrades in vivo and can result in non-PEG components being released into the blood stream, which can be undesirable.
It is desirable to develop improved polymers providing additional choices for use in drug delivery and other applications.